C. difficile is the most important cause of nosocomial intestinal infections and is the major cause of pseudomembranous colitis in humans (Bartlett et al Am. J. Clin. Nutr. 11 suppl:2521-6 (1980)). The overall associated mortality rate for individuals infected with C. difficile was calculated to be 5.99% within 3 months of diagnosis, with higher mortality associated with advanced age, being 13.5% in patients over 80 years (Karas et al Journal of Infection 561:1-9 (2010)). The current treatment for C. difficile infection is the administration of antibiotics (metronidazole and vancomycin), however there has been evidence of strains which are resistant to these antibiotics (Shah et al., Expert Rev. Anti Infect. Ther. 8(5), 555-564 (2010)). Accordingly there is a need for immunogenic compositions capable of inducing antibodies to, and/or a protective immune response to, C. difficile. 
The enterotoxicity of C. difficile is primarily due to the action of two toxins, toxin A and toxin B. These are both potent cytotoxins (Lyerly et al Current Microbiology 21:29-32 (1990).
It has been demonstrated that fragments of toxin A, in particular fragments of the C-terminal domain, can lead to a protective immune response in hamsters (Lyerly et al Current Microbiology 21:29-32 (1990)), WO96/12802 and WO00/61762. However the present inventors have demonstrated that antibodies against toxin A and toxin B alone are not sufficient in order to prevent disease caused by certain strains, in particular serogroup 078 and 027 strains. For this reason vaccines which are capable of protecting against these strains are still required.
Some strains, but not all, also express the binary toxin (CDT). Similar to many other binary toxins, CDT is composed of two components—an enzymatically active component (CDTa) and a catalytically inert transport component (CDTb). The catalytically inert component facilitates translocation of the CDTa into the target cell.
CDTa has an ADP-ribosylating activity, which transfers the ADP-ribose moiety of NAD/NADPH to the monomeric actin (G-actin) in the target cell and thus preventing its polymerization to F-actin and resulting in disruption of the cytoskeleton and eventual cell death (Sundriyal et al, Protein expression and Purification 74 (2010) 42-48).
WO2013/112867 (Merck) describes vaccines against Clostridium difficile comprising recombinant C. difficile Toxin A and Toxin B and binary toxin A (CDTa) proteins comprising specifically defined mutations relative to the native toxin sequence that are described as substantially reducing or eliminating toxicity, in combination with binary toxin B (CDTb).
The present inventors have found, that binary toxin can be used to provide an improved vaccine against C. difficile particularly providing protection against several of the most concerning C. difficile strains (such as the 027 and 078 strains). Furthermore the present inventors have demonstrated, for the first time, that only CDTa or CDTb (not both) is required in order to generate antibodies which are capable of neutralizing strains expressing binary toxin. In addition the inventors have demonstrated, for the first time, that CDTa proteins comprising mutations which reduce the ADP-ribosylating activity of CDTa, are still capable of raising an immune response. In addition, the inventors have demonstrated that truncated CDTa proteins are capable of raising an immune response. Similarly the inventors have demonstrated that truncated CDTb proteins are capable of raising an immune response, that CDTb can raise an immune response when it is in its monomeric or polymeric form and that fusion proteins comprising CDTa and CDTb or CDTb fused to isolated toxin A and/or isolated toxin B are capable of raising an immune response. Finally, the inventors have demonstrated that an immunogenic composition comprising binary toxin can be improved by adding an adjuvant, in particular an adjuvant comprising an immunologically active saponin presented in the form of a liposome or an oil in water emulsion.